<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html lang="en" dir="ltr" class="client-nojs">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Object - cppreference.com</title>
<meta charset="UTF-8">
<meta name="generator" content="MediaWiki 1.21.2">


<link rel="shortcut icon" href="../../../favicon.ico">



<link rel="stylesheet" href="../../../common/ext.css">
<meta name="ResourceLoaderDynamicStyles" content="">
<link rel="stylesheet" href="../../../common/site_modules.css">
<style>a:lang(ar),a:lang(ckb),a:lang(fa),a:lang(kk-arab),a:lang(mzn),a:lang(ps),a:lang(ur){text-decoration:none}#toc{display:none}.editsection{display:none}
/* cache key: mwiki1-mwiki_en_:resourceloader:filter:minify-css:7:472787eddcf4605d11de8c7ef047234f */</style>

<script src="../../../common/startup_scripts.js"></script>
<script>if(window.mw){
mw.config.set({"wgCanonicalNamespace":"","wgCanonicalSpecialPageName":false,"wgNamespaceNumber":0,"wgPageName":"cpp/language/object","wgTitle":"cpp/language/object","wgCurRevisionId":68998,"wgArticleId":13083,"wgIsArticle":true,"wgAction":"view","wgUserName":null,"wgUserGroups":["*"],"wgCategories":[],"wgBreakFrames":false,"wgPageContentLanguage":"en","wgSeparatorTransformTable":["",""],"wgDigitTransformTable":["",""],"wgDefaultDateFormat":"dmy","wgMonthNames":["","January","February","March","April","May","June","July","August","September","October","November","December"],"wgMonthNamesShort":["","Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"],"wgRelevantPageName":"cpp/language/object","wgRestrictionEdit":[],"wgRestrictionMove":[]});
}</script><script>if(window.mw){
mw.loader.implement("user.options",function(){mw.user.options.set({"ccmeonemails":0,"cols":80,"date":"default","diffonly":0,"disablemail":0,"disablesuggest":0,"editfont":"default","editondblclick":0,"editsection":0,"editsectiononrightclick":0,"enotifminoredits":0,"enotifrevealaddr":0,"enotifusertalkpages":1,"enotifwatchlistpages":0,"extendwatchlist":0,"externaldiff":0,"externaleditor":0,"fancysig":0,"forceeditsummary":0,"gender":"unknown","hideminor":0,"hidepatrolled":0,"imagesize":2,"justify":0,"math":1,"minordefault":0,"newpageshidepatrolled":0,"nocache":0,"noconvertlink":0,"norollbackdiff":0,"numberheadings":0,"previewonfirst":0,"previewontop":1,"quickbar":5,"rcdays":7,"rclimit":50,"rememberpassword":0,"rows":25,"searchlimit":20,"showhiddencats":0,"showjumplinks":1,"shownumberswatching":1,"showtoc":0,"showtoolbar":1,"skin":"cppreference2","stubthreshold":0,"thumbsize":2,"underline":2,"uselivepreview":0,"usenewrc":0,"watchcreations":0,"watchdefault":0,"watchdeletion":0,
"watchlistdays":3,"watchlisthideanons":0,"watchlisthidebots":0,"watchlisthideliu":0,"watchlisthideminor":0,"watchlisthideown":0,"watchlisthidepatrolled":0,"watchmoves":0,"wllimit":250,"variant":"en","language":"en","searchNs0":true,"searchNs1":false,"searchNs2":false,"searchNs3":false,"searchNs4":false,"searchNs5":false,"searchNs6":false,"searchNs7":false,"searchNs8":false,"searchNs9":false,"searchNs10":false,"searchNs11":false,"searchNs12":false,"searchNs13":false,"searchNs14":false,"searchNs15":false,"gadget-ColiruCompiler":1});;},{},{});mw.loader.implement("user.tokens",function(){mw.user.tokens.set({"editToken":"+\\","patrolToken":false,"watchToken":false});;},{},{});
/* cache key: mwiki1-mwiki_en_:resourceloader:filter:minify-js:7:ca03345b1e2c4d90a25d968753a73b92 */
}</script>
<script>if(window.mw){
mw.loader.load(["mediawiki.page.startup","mediawiki.legacy.wikibits","mediawiki.legacy.ajax"]);
}</script>
<style type="text/css">/*<![CDATA[*/
.source-cpp {line-height: normal;}
.source-cpp li, .source-cpp pre {
	line-height: normal; border: 0px none white;
}
/**
 * GeSHi Dynamically Generated Stylesheet
 * --------------------------------------
 * Dynamically generated stylesheet for cpp
 * CSS class: source-cpp, CSS id: 
 * GeSHi (C) 2004 - 2007 Nigel McNie, 2007 - 2008 Benny Baumann
 * (http://qbnz.com/highlighter/ and http://geshi.org/)
 * --------------------------------------
 */
.cpp.source-cpp .de1, .cpp.source-cpp .de2 {font: normal normal 1em/1.2em monospace; margin:0; padding:0; background:none; vertical-align:top;}
.cpp.source-cpp  {font-family:monospace;}
.cpp.source-cpp .imp {font-weight: bold; color: red;}
.cpp.source-cpp li, .cpp.source-cpp .li1 {font-weight: normal; vertical-align:top;}
.cpp.source-cpp .ln {width:1px;text-align:right;margin:0;padding:0 2px;vertical-align:top;}
.cpp.source-cpp .li2 {font-weight: bold; vertical-align:top;}
.cpp.source-cpp .kw1 {color: #0000dd;}
.cpp.source-cpp .kw2 {color: #0000ff;}
.cpp.source-cpp .kw3 {color: #0000dd;}
.cpp.source-cpp .kw4 {color: #0000ff;}
.cpp.source-cpp .co1 {color: #909090;}
.cpp.source-cpp .co2 {color: #339900;}
.cpp.source-cpp .coMULTI {color: #ff0000; font-style: italic;}
.cpp.source-cpp .es0 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es1 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es2 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es3 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es4 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es5 {color: #008000; font-weight: bold;}
.cpp.source-cpp .br0 {color: #008000;}
.cpp.source-cpp .sy0 {color: #008000;}
.cpp.source-cpp .sy1 {color: #000080;}
.cpp.source-cpp .sy2 {color: #000040;}
.cpp.source-cpp .sy3 {color: #000040;}
.cpp.source-cpp .sy4 {color: #008080;}
.cpp.source-cpp .st0 {color: #008000;}
.cpp.source-cpp .nu0 {color: #000080;}
.cpp.source-cpp .nu6 {color: #000080;}
.cpp.source-cpp .nu8 {color: #000080;}
.cpp.source-cpp .nu12 {color: #000080;}
.cpp.source-cpp .nu16 {color:#000080;}
.cpp.source-cpp .nu17 {color:#000080;}
.cpp.source-cpp .nu18 {color:#000080;}
.cpp.source-cpp .nu19 {color:#000080;}
.cpp.source-cpp .ln-xtra, .cpp.source-cpp li.ln-xtra, .cpp.source-cpp div.ln-xtra {background-color: #ffc;}
.cpp.source-cpp span.xtra { display:block; }

/*]]>*/
</style>
<style type="text/css">/*<![CDATA[*/
.source-text {line-height: normal;}
.source-text li, .source-text pre {
	line-height: normal; border: 0px none white;
}
/**
 * GeSHi Dynamically Generated Stylesheet
 * --------------------------------------
 * Dynamically generated stylesheet for text
 * CSS class: source-text, CSS id: 
 * GeSHi (C) 2004 - 2007 Nigel McNie, 2007 - 2008 Benny Baumann
 * (http://qbnz.com/highlighter/ and http://geshi.org/)
 * --------------------------------------
 */
.text.source-text .de1, .text.source-text .de2 {font: normal normal 1em/1.2em monospace; margin:0; padding:0; background:none; vertical-align:top;}
.text.source-text  {font-family:monospace;}
.text.source-text .imp {font-weight: bold; color: red;}
.text.source-text li, .text.source-text .li1 {font-weight: normal; vertical-align:top;}
.text.source-text .ln {width:1px;text-align:right;margin:0;padding:0 2px;vertical-align:top;}
.text.source-text .li2 {font-weight: bold; vertical-align:top;}
.text.source-text .ln-xtra, .text.source-text li.ln-xtra, .text.source-text div.ln-xtra {background-color: #ffc;}
.text.source-text span.xtra { display:block; }

/*]]>*/
</style>
<!--[if lt IE 7]><style type="text/css">body{behavior:url("/mwiki/skins/cppreference2/csshover.min.htc")}</style><![endif]-->
</head>
<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_object skin-cppreference2 action-view cpp-navbar">
        <!-- header -->
        
        <!-- /header -->
        <!-- content -->
        <div id="cpp-content-base">
            <div id="content">
                <a id="top"></a>
                <div id="mw-js-message" style="display:none;"></div>
                                <!-- firstHeading -->
                <h1 id="firstHeading" class="firstHeading">Object</h1>
                <!-- /firstHeading -->
                <!-- bodyContent -->
                <div id="bodyContent">
                                        <!-- tagline -->
                    <div id="siteSub">From cppreference.com</div>
                    <!-- /tagline -->
                                        <!-- subtitle -->
                    <div id="contentSub"><span class="subpages">&lt; <a href="../../cpp.html" title="cpp">cpp</a>‎ | <a href="../language.1.html" title="cpp/language">language</a></span></div>
                    <!-- /subtitle -->
                                                            <!-- bodycontent -->
                    <div id="mw-content-text" lang="en" dir="ltr" class="mw-content-ltr">
<div class="t-navbar" style="">
<div class="t-navbar-sep"> </div>
<div class="t-navbar-head">
<a href="../../cpp.html" title="cpp"> C++</a><div class="t-navbar-menu"><div>
<div><table class="t-nv-begin" cellpadding="0" style="line-height:1.1em;">
<tr class="t-nv"><td colspan="5"> <a href="../language.1.html" title="cpp/language"> Language</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../header.html" title="cpp/header"> Standard library headers</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../concept.html" title="cpp/concept"> Concepts</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../utility.html" title="cpp/utility"> Utilities library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../string.html" title="cpp/string"> Strings library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../container.html" title="cpp/container"> Containers library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../algorithm.html" title="cpp/algorithm"> Algorithms library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../iterator.html" title="cpp/iterator"> Iterators library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../numeric.html" title="cpp/numeric"> Numerics library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../io.html" title="cpp/io"> Input/output library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../locale.html" title="cpp/locale"> Localizations library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../regex.html" title="cpp/regex"> Regular expressions library</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../atomic.html" title="cpp/atomic"> Atomic operations library</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../thread.html" title="cpp/thread"> Thread support library</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
</table></div>
<div></div>
</div></div>
</div>
<div class="t-navbar-sep"> </div>
<div class="t-navbar-head"><a href="../language.1.html" title="cpp/language"> C++ language</a></div>
<div class="t-navbar-sep"> </div>
<div class="t-navbar-head">
<a href="basic_concepts.html" title="cpp/language/basic concepts"> Basic Concepts</a><div class="t-navbar-menu"><div>
<div style="display:inline-block">
<div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="../keywords.html" title="cpp/keyword"> language keywords</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="translation_phases.html" title="cpp/language/translation phases"> phases of translation</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../comment.html" title="cpp/comment"> comments </a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="main_function.html" title="cpp/language/main function"> the main() function</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="name.html" title="cpp/language/identifiers"> names and identifiers</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="incomplete_type.html" title="cpp/language/type"> types</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="types.html" title="cpp/language/types"> arithmetic types</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink"> objects</strong> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="scope.html" title="cpp/language/scope"> scope</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="lifetime.html" title="cpp/language/lifetime"> object lifetime</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="storage_duration.html" title="cpp/language/storage duration"> storage duration and linkage</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="definition.html" title="cpp/language/definition"> definitions and ODR</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="lookup.html" title="cpp/language/lookup"> name lookup</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="memory_model.html" title="cpp/language/memory model"> memory model</a> </td></tr>
</table></div>
</div>
<div></div>
</div></div>
</div>
<div class="t-navbar-sep"> </div>
</div>
<p>C++ programs create, destroy, refer to, access, and manipulate <i>objects</i>
</p>
<p>An object, in C++, is a <i>region of storage</i> that has
</p>
<ul>
<li> size (can be determined with <a href="sizeof.html" title="cpp/language/sizeof">sizeof</a>)
</li>
<li> alignment requirement (can be determined by <a href="alignof.html" title="cpp/language/alignof">alignof</a>)
</li>
<li> <a href="storage_duration.html" title="cpp/language/storage duration">storage duration</a> (automatic, static, dynamic, thread-local)
</li>
<li> <a href="lifetime.html" title="cpp/language/lifetime">lifetime</a> (bounded by storage duration or temporary)
</li>
<li> <a href="incomplete_type.html" title="cpp/language/type">type</a>
</li>
<li> value (which may be indeterminate, e.g. for <a href="default_initialization.html" title="cpp/language/default initialization">default-initialized</a> non-class types)
</li>
<li> optionally, a <a href="name.html" title="cpp/language/name" class="mw-redirect">name</a>
</li>
</ul>
<p>Functions, references, classes and other types, namespaces, enumerators, and templates are not objects.
</p>
<p>Objects are created by <a href="definition.html" title="cpp/language/definition">definitions</a>, <a href="new.html" title="cpp/language/new">new-expressions</a>, and in several  other situations where temporary objects are required: <a href="reference_initialization.html" title="cpp/language/reference initialization">binding a reference to a prvalue</a>, returning a prvalue from a function, <a href="expressions.html%23Conversions" title="cpp/language/expressions">a conversion</a> that creates a prvalue, <a href="throw.html" title="cpp/language/throw">throwing</a> an exception, entering an exception handler, and in some <a href="initialization.html" title="cpp/language/initialization">initialization</a> contexts.
</p>
<table id="toc" class="toc"><tr><td>
<div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="objects.html%23Object_representation_and_value_representation"><span class="tocnumber">1</span> <span class="toctext">Object representation and value representation</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="objects.html%23Subobjects"><span class="tocnumber">2</span> <span class="toctext">Subobjects</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="objects.html%23Polymorphic_objects"><span class="tocnumber">3</span> <span class="toctext">Polymorphic objects</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="objects.html%23Strict_aliasing"><span class="tocnumber">4</span> <span class="toctext">Strict aliasing</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="objects.html%23Alignment"><span class="tocnumber">5</span> <span class="toctext">Alignment</span></a></li>
</ul>
</td></tr></table>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Object representation and value representation">edit</a>]</span> <span class="mw-headline" id="Object_representation_and_value_representation">Object representation and value representation</span>
</h3>
<p>For an object of type <code>T</code>, <i>object representation</i> is the sequence of <span class="t-c"><span class="mw-geshi cpp source-cpp">sizeof<span class="br0">(</span>T<span class="br0">)</span></span></span> objects of type <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span> beginning at the same address as the <code>T</code> object.
</p>
<p>The <i>value representation</i> of an object is the set of bits that hold the value of its type <code>T</code>.
</p>
<p>For <a href="../concept/TriviallyCopyable.html" title="cpp/concept/TriviallyCopyable"><code>TriviallyCopyable</code></a> types, value representation is a part of the object representation, which means that copying the bytes occupied by the object in the storage is sufficient to produce another object with the same value.
</p>
<p>Some bits of the object representation may not participate in the value representation; such bits may be padding introduced to satisfy <a href="objects.html%23Alignment" title="cpp/language/object">alignment requirements</a>, <a href="bit_field.html" title="cpp/language/bit field">bit field</a> sizes, etc.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;cassert&gt;</span>
<span class="kw1">struct</span> S <span class="br0">{</span>
    <span class="kw4">char</span> c<span class="sy4">;</span>  <span class="co1">// 1 byte value</span>
             <span class="co1">// 3 bytes padding</span>
    <span class="kw4">float</span> f<span class="sy4">;</span> <span class="co1">// 4 bytes value</span>
    <span class="kw4">bool</span> operator<span class="sy1">==</span><span class="br0">(</span><span class="kw4">const</span> S<span class="sy3">&amp;</span> arg<span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="co1">// value-based equality</span>
        <span class="kw1">return</span> c <span class="sy1">==</span> arg.<span class="me1">c</span> <span class="sy3">&amp;&amp;</span> f <span class="sy1">==</span> arg.<span class="me1">f</span><span class="sy4">;</span>
    <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
<a href="../error/assert.html"><span class="kw620">assert</span></a><span class="br0">(</span>sizeof<span class="br0">(</span>S<span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">8</span><span class="br0">)</span><span class="sy4">;</span>
S s1 <span class="sy1">=</span> <span class="br0">{</span><span class="st0">'a'</span>, <span class="nu16">3.14</span><span class="br0">}</span><span class="sy4">;</span>
S s2 <span class="sy1">=</span> s1<span class="sy4">;</span>
<span class="kw1">reinterpret_cast</span><span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy2">*</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="sy3">&amp;</span>s1<span class="br0">)</span><span class="br0">[</span><span class="nu0">2</span><span class="br0">]</span> <span class="sy1">=</span> <span class="st0">'b'</span><span class="sy4">;</span> <span class="co1">// change 2nd byte</span>
<a href="../error/assert.html"><span class="kw620">assert</span></a><span class="br0">(</span>s1 <span class="sy1">==</span> s2<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// value did not change</span></pre></div></div>
<p>For the objects of type <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">char</span></span></span>, <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">signed</span> <span class="kw4">char</span></span></span>, and <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span>, every bit of the object representation is required to participate in the value representation (no padding allowed)
</p>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Subobjects">edit</a>]</span> <span class="mw-headline" id="Subobjects">Subobjects</span>
</h3>
<p>An object can contain other objects, which are called <i>subobjects</i>. These include
</p>
<ul>
<li> member objects
</li>
<li> base class subobjects
</li>
<li> array elements
</li>
</ul>
<p>An object that is not a subobject of another object is called <i>complete object</i>
</p>
<p>Complete objects, member objects, and array elements are also known as <i>most derived objects</i>, to distinguish them from base class subobjects. The size of a <i>most derived object</i> that is not a <a href="bit_field.html" title="cpp/language/bit field">bit field</a> is required to be non-zero (the size of a base class subobject may be zero: see <a href="ebo.html" title="cpp/language/ebo">empty base optimization</a>)
</p>
<p>Any two objects (that are not <a href="bit_field.html" title="cpp/language/bit field">bit fields</a>) are guaranteed to have different addresses unless one of them is a subobject of another, or if they are subobjects of different type within the same complete object, and one of them is a zero-size base.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">char</span> c1 <span class="sy1">=</span> ’x’<span class="sy4">;</span>
<span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">char</span> c2 <span class="sy1">=</span> ’x’<span class="sy4">;</span>
<a href="../error/assert.html"><span class="kw620">assert</span></a><span class="br0">(</span><span class="sy3">&amp;</span>c1 <span class="sy3">!</span><span class="sy1">=</span> <span class="sy3">&amp;</span>c2<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// same values, different addresses</span></pre></div></div>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Polymorphic objects">edit</a>]</span> <span class="mw-headline" id="Polymorphic_objects">Polymorphic objects</span>
</h3>
<p>Objects of class type that declare or inherit at least one virtual function are polymorphic objects. Within each polymorphic object, the implementation stores additional information (in every existing implementation, it is one pointer unless optimized out), which is used by <a href="virtual.html" title="cpp/language/virtual">virtual function</a> calls and by the RTTI features (<a href="dynamic_cast.html" title="cpp/language/dynamic cast">dynamic_cast</a> and <a href="typeid.html" title="cpp/language/typeid">typeid</a>) to determine, at run time, the type with which the object was created, regardless of the expression it is used in.
</p>
<p>For non-polymorphic objects, the interpretation of the value is determined from the expression in which the object is used, and is decided at compile time.
</p>
<div class="t-example">
<div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;typeinfo&gt;</span>
<span class="kw1">struct</span> Base1 <span class="br0">{</span>
    <span class="co1">// polymorphic type: declares a virtual member</span>
    <span class="kw1">virtual</span> ~Base1<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> Derived1 <span class="sy4">:</span> Base1 <span class="br0">{</span>
     <span class="co1">// polymorphic type: inherits a virtual member</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> Base2 <span class="br0">{</span>
     <span class="co1">// non-polymorphic type</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> Derived2 <span class="sy4">:</span> Base2 <span class="br0">{</span>
     <span class="co1">// non-polymorphic type</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    Derived1 obj1<span class="sy4">;</span> <span class="co1">// object1 created with type Derived1</span>
    Derived2 obj2<span class="sy4">;</span> <span class="co1">// object2 created with type Derived2</span>
 
    Base1<span class="sy3">&amp;</span> b1 <span class="sy1">=</span> obj1<span class="sy4">;</span> <span class="co1">// b1 refers to the object obj1</span>
    Base2<span class="sy3">&amp;</span> b2 <span class="sy1">=</span> obj2<span class="sy4">;</span> <span class="co1">// b2 refers to the object obj2</span>
 
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"Expression type of b1: "</span> <span class="sy1">&lt;&lt;</span> <span class="kw1">typeid</span><span class="br0">(</span>decltype<span class="br0">(</span>b1<span class="br0">)</span><span class="br0">)</span>.<span class="me1">name</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span>
              <span class="sy1">&lt;&lt;</span> <span class="st0">"Expression type of b2: "</span> <span class="sy1">&lt;&lt;</span> <span class="kw1">typeid</span><span class="br0">(</span>decltype<span class="br0">(</span>b2<span class="br0">)</span><span class="br0">)</span>.<span class="me1">name</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span>
              <span class="sy1">&lt;&lt;</span> <span class="st0">"Object type of b1: "</span> <span class="sy1">&lt;&lt;</span> <span class="kw1">typeid</span><span class="br0">(</span>b1<span class="br0">)</span>.<span class="me1">name</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span>
              <span class="sy1">&lt;&lt;</span> <span class="st0">"Object type of b2: "</span> <span class="sy1">&lt;&lt;</span> <span class="kw1">typeid</span><span class="br0">(</span>b2<span class="br0">)</span>.<span class="me1">name</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span>
              <span class="sy1">&lt;&lt;</span> <span class="st0">"size of b1: "</span> <span class="sy1">&lt;&lt;</span> sizeof b1 <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span>
              <span class="sy1">&lt;&lt;</span> <span class="st0">"size of b2: "</span> <span class="sy1">&lt;&lt;</span> sizeof b2 <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">Expression type of b1: Base1 Expression type of b2: Base2
Object type of b1: Derived1 Object type of b2: Base2
size of b1: 8 size of b2: 1</pre></div></div> 
</div>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Strict aliasing">edit</a>]</span> <span class="mw-headline" id="Strict_aliasing">Strict aliasing</span>
</h3>
<p>Accessing an object using an expression of a type other than the type which which it was created is undefined behavior in many cases, see <a href="reinterpret_cast.html%23Type_aliasing" title="cpp/language/reinterpret cast">reinterpret_cast</a> for the list of exceptions and examples.
</p>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Alignment">edit</a>]</span> <span class="mw-headline" id="Alignment">Alignment</span>
</h3>
<p>Every <a href="incomplete_type.html" title="cpp/language/type">object type</a> has the property called <i>alignment requirement</i>, which is an integer value (of type <span class="t-lc"><a href="../types/size_t.html" title="cpp/types/size t">std::size_t</a></span>, always a power of 2) representing the number of bytes between successive addresses at which objects of this type can be allocated. The alignment requirement of a type can be queried with <a href="alignof.html" title="cpp/language/alignof">alignof</a> or <span class="t-lc"><a href="../types/alignment_of.html" title="cpp/types/alignment of">std::alignment_of</a></span>. The pointer alignment function <span class="t-lc"><a href="../memory/align.html" title="cpp/memory/align">std::align</a></span> can be used to obtain a suitably-aligned pointer within some buffer, and <span class="t-lc"><a href="../types/aligned_storage.html" title="cpp/types/aligned storage">std::aligned_storage</a></span> can be used to obtain suitably-aligned storage.
</p>
<p>Each object type imposes its alignment requirement on every object of that type; stricter alignment (with larger alignment requirement) can be requested using <a href="alignas.html" title="cpp/language/alignas">alignas</a>.
</p>
<p>In order to satisfy alignment requirements of all non-static members of a <a href="class.html" title="cpp/language/class">class</a>, <i>padding</i> may be inserted after some of its members.
</p>
<div class="t-example">
<div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
 
<span class="co1">// objects of type S can be allocated at any address</span>
<span class="co1">// because both S.a and S.b can be allocated at any address</span>
<span class="kw1">struct</span> S <span class="br0">{</span>
  <span class="kw4">char</span> a<span class="sy4">;</span> <span class="co1">// size: 1, alignment: 1</span>
  <span class="kw4">char</span> b<span class="sy4">;</span> <span class="co1">// size: 1, alignment: 1</span>
<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// size: 2, alignment: 1</span>
 
<span class="co1">// objects of type X must be allocated at 4-byte boundaries</span>
<span class="co1">// because X.n must be allocated at 4-byte boundaries</span>
<span class="co1">// because int's alignment requirement is (usually) 4</span>
<span class="kw1">struct</span> X <span class="br0">{</span>
  <span class="kw4">int</span> n<span class="sy4">;</span>  <span class="co1">// size: 4, alignment: 4</span>
  <span class="kw4">char</span> c<span class="sy4">;</span> <span class="co1">// size: 1, alignment: 1</span>
  <span class="co1">// three bytes padding</span>
<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// size: 8, alignment: 4 </span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"sizeof(S) = "</span> <span class="sy1">&lt;&lt;</span> sizeof<span class="br0">(</span>S<span class="br0">)</span>
              <span class="sy1">&lt;&lt;</span> <span class="st0">" alignof(S) = "</span> <span class="sy1">&lt;&lt;</span> alignof<span class="br0">(</span>S<span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"sizeof(X) = "</span> <span class="sy1">&lt;&lt;</span> sizeof<span class="br0">(</span>X<span class="br0">)</span>
              <span class="sy1">&lt;&lt;</span> <span class="st0">" alignof(X) = "</span> <span class="sy1">&lt;&lt;</span> alignof<span class="br0">(</span>X<span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">sizeof(S) = 2 alignof(S) = 1
sizeof(X) = 8 alignof(X) = 4</pre></div></div> 
</div>
<p>The weakest alignment (the smallest alignment requirement) is the alignment of <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">char</span></span></span> (typically 1), the largest <i>fundamental alignment</i> of any type is the alignment of <span class="t-lc"><a href="../types/max_align_t.html" title="cpp/types/max align t">std::max_align_t</a></span>. If a type's alignment is made stricter (larger) than <span class="t-lc"><a href="../types/max_align_t.html" title="cpp/types/max align t">std::max_align_t</a></span> using <a href="alignas.html" title="cpp/language/alignas">alignas</a>, it is known as a type with <i>extended alignment</i> requirement. A type whose alignment is extended or a class type whose non-static data member has extended alignment is an <i>over-aligned type</i>. It is implementation-defined if <a href="new.html" title="cpp/language/new">new-expression</a>, <span class="t-lc"><a href="../memory/allocator/allocate.html" title="cpp/memory/allocator/allocate">std::allocator::allocate</a></span>, and <span class="t-lc"><a href="../memory/get_temporary_buffer.html" title="cpp/memory/get temporary buffer">std::get_temporary_buffer</a></span> support over-aligned types. <a href="../concept/Allocator.html" title="cpp/concept/Allocator"><code>Allocator</code>s</a> instantiated with over-aligned types are allowed to fail to instantiate at compile time, to throw <span class="t-lc"><a href="../memory/new/bad_alloc.html" title="cpp/memory/new/bad alloc">std::bad_alloc</a></span> at runtime, to silently ignore unsupported alignment requirement, or to handle them correctly.
</p>
<!-- 
NewPP limit report
Preprocessor visited node count: 1249/1000000
Preprocessor generated node count: 3011/1000000
Post‐expand include size: 26911/2097152 bytes
Template argument size: 9646/2097152 bytes
Highest expansion depth: 13/40
Expensive parser function count: 0/100
-->

<!-- Saved in parser cache with key mwiki1-mwiki_en_:pcache:idhash:13083-0!*!0!!en!*!* and timestamp 20140323203531 -->
</div>                    <!-- /bodycontent -->
                                        <!-- printfooter -->
                    <div class="printfooter">
                    Retrieved from "<a href="http%3A//en.cppreference.com/mwiki/index.php">http://en.cppreference.com/mwiki/index.php?title=cpp/language/object&amp;oldid=68998</a>"                    </div>
                    <!-- /printfooter -->
                                                            <!-- catlinks -->
                    <div id="catlinks" class="catlinks catlinks-allhidden"></div>                    <!-- /catlinks -->
                                                            <div class="visualClear"></div>
                    <!-- debughtml -->
                                        <!-- /debughtml -->
                </div>
                <!-- /bodyContent -->
            </div>
        </div>
        <!-- /content -->
        <!-- footer -->
        
        <!-- /footer -->
        <script>if(window.mw){
mw.loader.state({"site":"loading","user":"missing","user.groups":"ready"});
}</script>
<script src="../../../common/skin_scripts.js"></script>
<script>if(window.mw){
mw.loader.load(["mediawiki.action.view.postEdit","mediawiki.user","mediawiki.page.ready","mediawiki.searchSuggest","mediawiki.hidpi","ext.gadget.ColiruCompiler"], null, true);
}</script>
<script src="../../../common/site_scripts.js"></script>

<script type="text/javascript">
var _gaq = _gaq || [];
_gaq.push(['_setAccount', 'UA-2828341-1']);
_gaq.push(['_setDomainName', 'cppreference.com']);
_gaq.push(['_trackPageview']);
</script><!-- Served in 0.080 secs. -->
	</body>
<!-- Cached 20140323235607 -->
</html>
